TEMBLOR AND MONTEREY FORMATIONS 



73 



different aspect. The lowest of these are widely 

 spaced and thin, but within a hundred feet strati- 

 graphically they become so numerous as to dominate 

 the sequence. This medium-grained sandstone is a 

 friable limonite-rich sand that forms faintly laminated 

 beds about 5 feet thick. It crops out in only a few places, 

 and where it is exposed in the ridge south of Mine 

 Hill, it is deeply weathered. Quartz, orthoclase, and 

 subordinate plagioclase in grains averaging about 0.5 

 mm in diameter are the predominant minerals, but the 

 rock also contains a little muscovite and glauconite. 

 It has a weak cement apparently consisting of limo- 

 nite-stained clay; and oxidized grains of magnetite 

 and hematite seen under the microscope offer a pos- 

 sible source for the ocherous iron oxides. The sand- 

 stone is not highly fossiliferous, but inch-long echinoid 

 spines are conspicuous in some layers, and in others 

 a few well-preserved molds were found. These were 

 identified by Miss Myra Keen, of Stanford University, 

 as formed by Pecten discus Conrad, Pecten andesonia 

 Arnold, and Natica sp. 



Above the medium-grained buff sandstone south of 

 Mine Hill lie interbedded organic shale, siltstone, and 

 glauconitic arkosic sandstone. The total thickness of 

 these beds may be as much as 400 feet, but they crop 

 out in only a few places and are well exposed only 

 along the road leading eastward from the junction 

 of Guadalupe and Eincon Creeks. In the roadcuts 

 the organic shales and siltstones form well-defined 

 persistent beds up to 4 feet in thickness. Where fresh, 

 they are light to dark gray and flecked with minute 

 white spots, but they are mostly weathered to a gray- 

 ish brown or chocolate color. They are fairly mas- 

 sive and break along irregular or conchoidal frac- 

 tures as readily as along bedding planes. The larger 

 mineral grains are of quartz, orthoclase, plagioclase, 

 microline, and a little calcite, but a large part of the 

 rock is clay and organic matter. Foraminifera are 

 abundant, and fish scales are not uncommon. The 

 interbedded arkosic sandstone, occurring in beds of 

 comparable thickness, is a fine- to medium-grained 

 greenish-gray rock characterized by a relatively high 

 percent of glauconite and calcite cement. A few of 

 the thinner beds contain enough calcite to be termed 

 "arenaceous limestone." 



In the Blossom Hill area, and farther east along 

 the north edge of Los Capitancillos Ridge, the upper 

 part of the Temblor formation consists mainly of 

 light-colored well-sorted medium-grained sandstone; 

 but for about 4 miles eastward from a point a little 

 west of Guadalupe Canyon, the formation includes a 

 comparatively thin but mappable bed of dacitic tuff- 

 breccia. (See pi. 1.) This bed deserves particular 



686-671 O 63 6 



attention because of the importance attributed it by 

 Becker (1888, p. 314, 468), who believed that it was 

 an intrusive dike, probably of late Pliocene age, and 

 that it was the source of the solutions that deposited 

 the quicksilver ore. 



The dacitic tuff-breccia has a maximum thickness of 

 about 40 feet north of the Senator mine, but it thins 

 along its strike both eastward and westward to a 

 thickness of only a few feet. It is more resistant 

 to erosion than the Miocene sedimentary rocks, and 

 along much of its length it forms an irregular scarp 

 on the steep side of a hogback. Its outcrops are 

 rugged and irregularly rounded, and they give rise 

 to bouldery float that is strewn over the slope below. 

 In general, they are mostly white and streaked or 

 mottled with red and yellow iron stains; their sur- 

 faces are both pitted and embossed. In detail the 

 tuff-breccia shows considerable variation from place 

 to place, but it is everywhere fragmental. The frag- 

 ments are mostly angular and less than 1 inch in 

 diameter, but some are lenticular and of greater size; 

 these are generally less than 2 inches long, but some 

 are as much as 1 foot long. The lenticular fragments 

 are oriented parallel to the bedding and are rudely 

 banded parallel to their length; they are believed to 

 be collapsed pumice. Most of the larger angular frag- 

 ments are composed of volcanic rocks, but some of 

 them, particularly in the lower part of the tuff-brec- 

 cia, are sandstone and shale. The matrix material is 

 so altered in most places that its original nature can- 

 not be determined, but, largely because of the abun- 

 dance of volcanic fragments in the rock, it is believed 

 to be pyroclastic. If it is, the original tuffaceous ma- 

 terial must have contained many small, whole crystals 

 of plagioclase and some quartz, for locally, especially 

 where the rock is silicified, the rock resembles a fine- 

 grained dacitic lava. Alteration of the rock has 

 everywhere been intense. The ferromagnesian min- 

 erals have been completely leached and locally replaced 

 by jarosite; the feldspars in some areas were com- 

 pletely leached out, whereas in others they remained 

 fresh and glassy; and the quartz generally escaped 

 alteration. The alteration process was accompanied 

 by the filling of veins with quartz, opal, and jarosite. 



The tuff-breccia is believed to be a pyroclastic de- 

 posit rather than either a dike or a flow for the fol- 

 lowing reasons: 



1. It contains angular fragments throughout. 



2. It is underlain by somewhat tuffaceous sandstone, 



and locally grades upward through tuffaceous 

 sandstone to normal sandstone of the Temblor 

 formation. 



